1. Field of the Invention
This invention relates to a method for producing arylethylene. More particularly, the invention relates to a method for economically producing highly pure arylethylene, preferably p-isobutylstyrene, etc. in an industrial scale. Furthermore, the present invention relates to a method for producing highly pure arylethylene which comprises the steps of: cracking diarylethane, separating the cracking products, hydrogenating the recovered fraction mainly containing unreacted diarylethane, and cracking again the recovered hydrogenated diarylethane fraction generally by recycling it to the above cracking step. A preferable compound that is prepared according to the method of the present invention is p-isobutylstyrene. This compound is well known as a useful intermediate material for preparing ibuprofen (tradename, .alpha.-(4-isobutylphenyl)propionic acid) which is a medicine used for the relief of fever, pain and inflammation.
2. Description of the Prior Art
In connection with the preparation of arylethylene such as p-isobutylstyrene by cracking 1,1-diarylethane in the presence of an acid catalyst, various kinds of methods have been hitherto proposed. For example,
Ind. Eng. Chem., Vol. 46, No. 4, 652 (1954), PA1 J. Chem. Eng. Data, Vol. 9, No. 1, 104 (1964) and PA1 I & EC Prod. Res. Dev., Vol. 3, No. 1, 16 (1964). PA1 General formula (A): Ar.sub.1 --CH(CH.sub.3)--Ar.sub.2 PA1 General formula (B): Ar.sub.1 --CH.dbd.CH.sub.2 PA1 General formula (C): Ar.sub.2 --CH.dbd.CH.sub.2 PA1 General formula (D): Ar.sub.1 --H PA1 General formula (E): Ar.sub.2 --H
It is disclosed in the above references that alkylstyrenes such as methylstyrene and dimethylstyrene are obtained by cracking 1,1-diarylethanes such as 1,1-ditolylethane and 1,1-dixylylethane. Furthermore, they refer to the preparation of ethylstyrene, isopropylstyrene and tert-butylstyrene.
There are more definite references on the improvement in cracking catalysts:
U.S. Pat. No. 2,420,689: Method for preparing dimethylstyrene by cracking dixylylethane in the presence of kaolin catalyst
U.S. Pat. No. 2,422,318: Method for cracking asymmetric diarylethanes
U.S. Pat. No. 2,864,872: Method for using silica as a cracking catalyst
U.S. Pat. No. 2,954,413: Method for cracking dixylylethane using fluidized catalyst
U.S. Pat. No. 3,025,330: Method for preparing methylstyrene from ditolylethane
U.S. Pat. Nos. 2,976,333 and 2,976,334: Method for improving cracking catalyst
In the cracking of 1,1-diarylethane, all the 1,1-diarylethane is not converted into arylethylene (alkylstyrene) and alkylbenzene but unreacted 1,1-diarylethane is inevitably contained in the reaction mixture. This fact is apparent from the description of the above references that the conversion per one pass is 40 to 60%. In other words, unreacted starting material as much as 60 to 40% remains in the reaction product.
This fact is the same in the case of the cracking of 1,1-bis(4-isobutylphenyl)ethane and it was found out by the inventors of this application that the average cracking rate is in the range of 40 to 60%. In other words, unreacted 1,1-bis(4-isobutylphenyl)ethane as much as 40 to 60% remains.
Accordingly, in order to prepare economically arylethylene by the cracking of 1,1-diarylethane, it is inevitable to reuse and crack again the unreacted 1,1-diarylethane. In other words, when the fraction mainly containing 1,1-diarylethane is separated from the reaction mixture and used again for cracking, the industrial applicability of the cracking reaction depends upon the possibility to obtain arylethylene having a purity and properties which are suitable for the purpose of industrial uses.
Incidentally, arylethylenes obtained by cracking 1,1-diarylethanes have various uses, including aforesaid medicine, such as:
industrially useful intermediates proposed in West German Offenlegungsschrift No. 2 325 302 and British Pat. No. 1,565,235; and
raw material for the synthesis of weatherproof polymer disclosed in Ind. Eng. Chem., Vol. 46, 652 (1954). Therefore, the proposal of economical preparation of arylethylene has been wanted.
The present inventors made investigation into the economical and industrial working of the cracking of 1,1-diarylethane. As a result, it was found out that, when the fraction mainly containing unreacted 1,1-diarylethane is simply cracked again, the deterioration of cracking catalyst is severe with the passage of time and the properties of obtained arylethylene are not satisfactory.
That is, the present inventors noticed as a result of the cracking of 1,1-diarylethane that diarylethylenes having boiling points close to that of 1,1-diarylethane and being difficultly separated, are generated, and that the fraction mainly containing 1,1-diarylethane cannot be prevented from containing the olefins. In addition, when this fraction is brought back to the cracking step and cracked again, because the material to be cracked contains diarylethylenes, complicated cracking product is obtained. Accordingly, a vicious circle is caused to occur in that the side reaction products of this re-cracking also have boiling points close to that of the aimed arylethylene, as a result, the contamination with the by-product in the aimed arylethylene fraction cannot be avoided. Therefore, it has not been possible to reuse a large quantity of unreacted 1,1-diarylethane fraction intact and thus, the conventional method for preparing arylethylene by cracking 1,1-diarylethane has not been economical method in view of industrial practice.